Method and device for noise reduction in a pulse-width controlled image display device

ABSTRACT

The invention relates to a method and a device for noise reduction in a pulse-width controlled image display device. A monitoring of whether the brightness of several sequential image points is brighter than a given brightness threshold value occurs. If not the case, the delayed but otherwise unchanged input signal is supplied to the display. If, on the other hand, the brightness of several sequential image points is less than the given threshold brightness value, a mean value generation for the brightness values of several sequential image points occurs and said mean value is supplied to the display.

BACKGROUND OF THE INVENTION

The invention concerns a method and a device for noise reduction in apulse-width-controlled image-display device.

For example, such a method and such a device are used in plasmadisplays, which will complement or replace the color tubes still used inhigh-quality television equipment at the present time. In connectionwith color tubes, the user of high-quality television equipment has beenaccustomed to flicker-free reproduction since the end of the 80s due tothe 100 Hz technology.

A plasma display is known from the journal, Radio Fernsehen ElektronikRFE, No. 2, 1997, pages 18–20, which consists of two glass plates withelectrodes arranged in a matrix, between which a noble gas mixture islocated. The image information in plasma displays is not represented bylines, as in the case of the cathode-ray tubes, but as a whole image.Since, in a plasma display, the individual image points cannot be turnedon and off individually at arbitrary times, the activation of the imagepoint must occur in an activation pass for the entire display.

The start-up of a plasma display occurs in several phases: anaddressing- or initialization phase, a holding- or activation phase anda turn-off phase.

In the addressing- or initialization phase, all cells of the plasmadisplay which are to be activated in the subsequent holding- oractivation phase are precharged. In the last step, the turn-off phase,the precharged cells are discharged and the image information is turnedoff.

The time interval available for the representation of a TV image isdivided into time intervals of different duration or differentweighting, while a predetermined activation sequence is chosen as afunction of the brightness of a particular image point. This correspondsto lighting up of the particular image point once or several timesduring the image presentation in the available time interval, where apredetermined time duration is assigned to each lighting up process.

Such known plasma displays are sold on the market, for example, by thecompanies Fujitsu and NEC.

From DE 198 33 597 A1, a method and a device are known for reduction offlicker in pulse-width-controlled image-display devices, especially in acolor plasma display. Such a color plasma display serves, for example,for the representation of TV images. The color plasma display iscontrolled with a pulse-width modulator, whereas, for triggering, theduration of a TV image is divided into a number of partial images orpartial time intervals, which are shown sequentially. In order to reduceflicker, especially to reduce a 50 Hz flicker, the sequence of thepartial time intervals and/or activation sequences of the partial timeintervals is predetermined so that the flickering of the images to bepresented is minimum.

Furthermore, a motion-detector-dependent change of the sequence of timeintervals is known. When motion is present, the sequence of timeintervals is chosen so that motion artifacts are avoided. Otherwise thesequence of the time intervals is such that 50 Hz flicker disturbancesare reduced.

Furthermore, it is already known in connection with plasma displays thatone can determine the brightness of an image to be represented, derive amaximum permissible illumination duration for each partial time intervalof the image to be represented from the determined brightness value andchange the maximum permissible illumination duration for each of thepartial time intervals in case of change of the detected brightnessvalue. This change is done in such a way that, at a determined darkimage content or low brightness value, the maximum permissibleillumination duration is increased in each of the partial time intervalsby the same time duration. However, on the other hand, if it is foundthat, in the brightness determination of the image to be represented, anoverall bright image content is present, then the maximum permissibleillumination duration of each partial time interval is reduced by aduration which is the same for all partial time intervals.

A disadvantage of this procedure consists in the fact that the contrastof the image to be produced is reduced, because, if the brightness ofthe image to be represented is high, the time duration for therepresentation of bright image components is reduced, and, if thebrightness of the image to be represented is low, dark gray imagecomponents are represented as light gray, because these are pulled up,that is, are lit up longer as a result of the application of a constantoffset described above.

Furthermore, a method is known from German Patent Application 100 09 858for improving the contrast of a pulse-width-controlled image displaydevice. In this method, a change of the maximum permissible illuminationduration for the partial time intervals is carried out in such a waythat the maximum permissible illumination duration is subjected tosmaller changes for partial time intervals with lower value than themaximum permissible illumination duration for partial time intervalswith a higher value.

SUMMARY OF THE INVENTION

Starting from this state of the art, the task of the invention is toprovide a new method and a new device in which noise is reduced.

The advantages of the invention consist in the fact that noise isreduced in the dark areas of a displayed image. Thus, the circumstanceis taken into consideration that, in the case of pulse-width-controlledimage-display devices, especially in plasma displays, the noise thatoccurs in the dark areas of the image is especially disturbing. This tobe attributed to the fact that short partial time intervals are assignedto dark image areas. For example, when, due to the noise, not only thepartial time interval is activated which is assigned to the LSB of amultibit word, but also the neighboring partial time interval LSB+1,then this means already a doubling of the brightness, which is reportedby the human eye and is found disturbing.

This applies especially when, in order to obtain artificial increase ofthe number of gray stages in the sense of a “dithering process”, bothLSB as well as LSB+1 are divided into four neighboring pixels of theimage to be shown. In this case, any noise present causes additionalspatial effects in the abovementioned four-block, which appearsdisturbing to the human eye.

With the aid of the claimed replacement of image points with lowbrightness values with average brightness values, the noise that occursin the dark areas of the image is reduced. In the bright areas of theimage, no change of the image signal occurs, since the noise in thebright areas of the image appears to be less disturbing.

The claimed method can be used both in the sense of a one-dimensionalfiltering as well as of a two-dimensional and even three-dimensionalfiltering. In the case of two-dimensional filtering, only additionalline memories, and in case of three-dimensional filtering, imagememories are additionally provided.

Optionally, the filtering that was performed can extend to image pointswhich lie in the transition region between filtered and unfiltered data.As a result of this, the transitions are made softer.

Furthermore, the number of sequential image points, which are checked inthe first step, can be chosen to be of different size. This also appliesto the number of image points which are used to form the mean value.Naturally, in these cases, the delay time of the delay member lying inthe path of the unfiltered signal must be adjusted appropriately.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantageous properties of the invention follow from theexplanation of a practical example with the aid of figures. Thefollowing are shown:

FIG. 1 is a sketch explaining the triggering of a pulse-width-controlledimage-display device,

FIG. 2 is a block diagram explaining the practical example of theinvention, and

FIG. 3 is a time diagram to illustrate the mode of functioning of theinvention.

DETAILED DESCRIPTION

FIG. 1 shows a sketch which explains the triggering of apulse-width-controlled image-display device, for example, of a plasmadisplay. The time interval of 20 milliseconds available for therepresentation of a TV image is divided into partial time intervals SF1,SF2, . . . , SF6. Each of these partial time intervals has a differentduration or weighting G, which are shown in FIG. 1, always above thelittle boxes representing the partial time intervals. During thesepartial time intervals, depending on the brightness value of aparticular image point, a predetermined activation sequence is selected.This corresponds to lighting up the image point once or several timesduring the time interval available for representing the image, and apredetermined time duration is assigned to each lighting up.

For example, in the case of dark image points, only one lighting upoccurs during the partial time interval SF1, which corresponds to theLSB of a video signal. If, due to noise, lighting up of SF2 also occursduring the partial time interval, which, in comparison to SF1, has alength which is approximately twice as long or weighted double, thenthis means an almost doubling of the brightness, which is detected bythe human eye as disturbing.

In order to avoid such disturbances caused by noise, which occur aboveall in dark areas of the image, a signal processing is performed as itis described in FIG. 2 with the aid of a practical example of theinvention.

FIG. 2 shows a block diagram to explain a practical example of theinvention. A video signal in the form of a signal data stream isintroduced to input E, which corresponds to the brightness signals ofsequential image points of the image to be displayed on the plasmadisplay. This video input signal is divided into three signal paths.

In the upper signal path, it arrives to a test unit 1, which checks ifthe brightness of three sequential image points is always smaller than apredetermined brightness threshold value. If this is not the case, thentest unit 1 generates a control signal for a switch 4 through whichswitch 4 is brought into its upper switching position. On the otherhand, if the brightness of three sequential image points is smaller thanthe predetermined brightness threshold value, then the test unit 1produces a control signal for switch 4 through which switch 4 is broughtinto its lower switching position.

In the middle signal path, a delay member 2 is provided in which theinput data stream is slowed down by the duration of two image points.The output signal of the delay member 2 is introduced to the upper inputof switch 4.

In the lower signal path, the input signal is passed through a filter 3,in which a linear or weighted mean value formation of three sequentialimage points occurs. The output signal of filter 3 is introduced to thelower input of switch 4.

The output of switch 4 is connected to a plasma display 5, where thesignal representation occurs.

When the brightness of three sequential image points are all below thebrightness threshold value, then, in the circuit shown, the lower signalpath is switched so that a mean signal is introduced to display 5 andthen, when the brightness of three sequential image points does not liebelow the brightness threshold value, the middle signal path isswitched, so that the delayed but otherwise unchanged input signal isfurther conducted to display 5.

The test described above and the signal processing performed as afunction of the test result, is repeated from image point to imagepoint.

FIG. 3 shows a time diagram in which the functioning of the invention isillustrated. In FIG. 3 a, the signal data stream present at the input Eis shown, where the brightness values of the sequential image points aregiven in the hexadecimal form. This signal data stream is subdividedinto three regions B1, B2, B3 for illustration, where the brightnessvalues of the image points in regions B1 and B3 are greater than thepredetermined brightness threshold value, which, in the presentpractical example, is at 30 in the hexadecimal representation and wherethe brightness values of the image points in area B2 are smaller thanthe abovementioned brightness threshold value.

FIG. 3 b shows the signal data stream present at the outlet of the delaymember 2, which is delayed in comparison to the input signal data streamby the duration of two image points.

FIG. 3 c shows the delayed, filtered data stream, as it is present atthe output of filter 3 and FIG. 3 d shows the data stream present at theoutput of switch 4. It can be seen from the latter that the brightnessvalues in the regions B1 and B3, apart from the delay by the duration oftwo image points, agree with the corresponding brightness values of theinput data stream. The brightness values in region B2 are, on the otherhand, replaced according to the present invention by mean values, whichwere determined by the brightness values of three sequential imagepoints.

By forming this mean value, which is performed only in the dark areas ofthe image, the noise in these dark image areas is reduced.

1. Method for noise reduction in a pulse-width-controlled image-displaydevice, wherein the time interval available for representing the imageis divided into sequential weighted partial time intervals, which form asignal data stream, and wherein the brightness signals present in theform of multibit words belonging to the image points of the image to berepresented, are produced by changing to activation sequences assignedto the partial time intervals, the method further comprising: a) in afirst step, the brightness of several sequential image points isexamined to determine if said brightness is greater than a predeterminedbrightness threshold value, b) in a second step then, when thebrightness values of several sequential image points are all smallerthan the predetermined brightness threshold value, a mean value of thebrightness value is formed from these brightness values for the severalsequential image points and this mean value is used in the signal datastream and then, when the brightness values of several sequential imagepoints are not all smaller than the predetermined brightness thresholdvalue, the corresponding multibit word is introduced in unchanged formto conversion in activation sequences; and c) the first and second stepis performed for each sequential image point; and d) wherein the numberof image points to be checked in the first step is at least three. 2.Method according to claim 1, wherein in the second step, when thebrightness values of several sequential image points are all smallerthan the predetermined brightness threshold value, a weighted mean valueformation of the brightness values of several sequential image points isperformed.
 3. A device for noise reduction in a pulse-width-controlledimage-display device, in which a time interval available forrepresenting an image is divided into sequential weighted partial timeintervals and the brightness signals that form a signal data stream,that belong to the image points of the image to be represented, and arein the form of multibit words, are produced by conversion intoactivation sequences assigned to the partial time intervals, the devicecomprising: an input connection (E) for receiving the signal data streamcorresponding to the brightness signals, a test unit (1) for checking ifthe brightness values of several sequential image points is smaller thanthe predetermined brightness threshold value, and a switch (4)controlled by the test unit, which, on the output side, when thebrightness values of several sequential image points are all not smallerthan the predetermined brightness threshold value, provides thetime-delayed signal data stream in the unchanged form and when thebrightness values of at least three sequential image points are allsmaller than the predetermined brightness threshold value, provides asignal data stream filtered in the sense of a mean value formation.